Recently, a light emitting device including a light emitting diode (LED) has been used as a backlight of a liquid crystal panel or a lighting device.
A driving circuit of an LED includes a DC/DC converter and a control circuit thereof. The control circuit controls a switching element of the DC/DC converter so that a current flowing through an LED bar (also referred to as an LED string) configured by connecting a plurality of LEDs in series approaches a target value. The brightness of the LEDs may be controlled by changing the target value (analog dimming).
In addition to this analog dimming, pulse width modulation (PWM) dimming may be used together in some cases. In the PWM dimming, a turn-on period during which a driving current is supplied to the LEDs and a turn-off period during which the driving current is interrupted are alternately repeated at constant intervals to change a time ratio of the turn-on period and the turn-off period, thereby changing the brightness of the LEDs.
In the DC/DC converter, there are various driving schemes. Conventionally, the designer of a liquid crystal display device has decided a driving scheme of the DC/DC converter and purchased a control circuit that supports the driving scheme.
However, a wide dynamic range is required for the backlight of the liquid crystal panel. In particular, in a driving circuit used in a display device that supports switching between 2D display and 3D display, it is necessary in a 3D display to make a driving current flow several times as much as a 2D display. That is, in the DC/DC converter of the driving circuit of the backlight LEDs, it is necessary to change its output current within the range of several times by the analog dimming.
In the DC/DC converter, there exist various characteristics such as power consumption (efficiency), heat generation, EMI characteristics, acoustic noise and the like. When the backlight LEDs are fixedly controlled by a single driving scheme as in the related art, there is a problem in that some characteristics are good, while others deteriorate, depending on a range of an output current.